There has been widely known several full penetration welding methods in which weld backing is used.
One of them is a method in which a weld backing made from copper is employed. In this method, a weld backing is first formed from copper, with its length identical to the length of the weld line of a workpiece to be welded and its configuration fitting to the shape of the weld line. Such a weld backing is held against the underside of the weld zone where a groove is formed and is fixed in position being supported by a number of supporting members. In this condition, a welder is moved along the weld line to weld throughout the entire length of the weld line. After completion of welding, the weld backing as well as its supporting members are removed.
Another method uses ceramic weld backings. According to this method, a number of ceramic backings of adequate shapes are fixedly placed along the entire weld line under the groove face of the weld zone. Then, a welder is moved to weld throughout the entire length of the weld line, and after completion of welding, the ceramic weld backings are all stripped off and removed.
These prior art methods present the following drawbacks. In the first method in which the weld backing made from copper or the like is used, the length of the weld backing should be the same as the length of the weld line. What is more, a weld backing having the same curvature as that of the weld line is required in the case of a bent weld line. Therefore, such a weld backing has no compatibility and as the weld line becomes longer, the cost of the material, processing and application increases.
Although the second method using ceramic weld backings is adaptive to a weld line in any conditions, the installation and removal of the backings by hand methods is indispensable, which is an obstacle to the realization of automated welding work. Further, the ceramic weld backings cannot be reused after removal, resulting in an increase in the cost. Another disadvantage of the second method is that if the workpiece is, for example, in the form of a box with a confined interior space such as a boom of a crane, backing work cannot be carried out.
Attempts have been made to overcome these drawbacks. For example, mobile weld backing positioners, in which a weld backing is mounted on a wheeled carriage and moved along a weld line, are disclosed in Japanese Patent Publication No. 53-20460 (1978) and in Japanese Patent Publication Laid Open No. 55-158899 (1980).
These publications teach the use of a backing tape and a backing board in combination as a weld backing. A wheeled carriage traveling on rails is provided with tension springs, links and other members. The backing board and the backing tape affixed to the upper face of the backing board are pressed against the underside of the groove face of a workpiece with a predetermined pressure, using the tension springs and other members. By mounting a backing on a wheeled carriage, the foregoing problems can be solved to some extent.
The mobile weld backing positioners disclosed in the above publications, however, require a backing tape to be interposed between a workpiece and the backing board, so that the cost inevitably increases. In addition, troublesome work for affixing the backing tape along a weld line and stripping it after completion of welding is necessary. This method cannot be applied when the space to be welded is too narrow to provide rails or to affix the backing tape. The use of the backing tape also makes it difficult to apply the method to welding of a corner joint.